1. Field of the Invention
The invention relates to a process allowing mixtures of trioxane and formaldehyde in the presence of water, defining a first trioxane:formaldehyde ratio, to be separated.
2. Background Art
A number of steps is required for the preparation of trioxane, which explains the large number of patents published.
Generally speaking, trioxane is prepared starting out from formaldehyde by a trimerization reaction in the presence of an acid catalyst. In the reaction, together with the trioxane, there remains unreacted formaldehyde and water, which must be appropriately removed.
Trioxane is obtained, according to the conventional process, starting from aqueous solutions of formaldehyde the concentration of which can vary between 30 and 70%, when raising them to a boil in the presence of an acid catalyst, which causes a trimerization reaction of the formaldehyde according to the following equation:3 CH2O<==>(CH2O)3
The extent of the reaction is highly dependent on the formaldehyde concentration in the solution and on the presence of trioxane therein. The greater the formaldehyde concentration and the smaller the trioxane content in the solution, the greater is the conversion.
The reaction takes place in a reactor and trioxane that is formed, in a relatively small amount due to the unfavorable balance, is vaporized together with the remaining components of the mixture and must be separated therefrom. The catalyst remains in the reactor, which receives, apart from the starting solution of formaldehyde, the residual currents of aqueous solutions of formaldehyde once this has been separated from the trioxane. This recirculating of residual currents of aqueous formaldehyde solutions lowers the yield of the reaction, because they dilute the formaldehyde solution contained in the reactor and they furthermore lower the conversion rate, since they are accompanied by a certain amount of trioxane that sets back the formation reaction thereof.
The vapors originated in the reactor pass to a distillation column at the head of which a mixture of trioxane, formaldehyde and water is obtained (that will hereinafter also be called an aqueous mixture of trioxane and formaldehyde, both if it is in the liquid phase and if it is in the vapor phase) in an approximate proportion of 30–40% trioxane, 17–30% formaldehyde and 40–50% water, which proportions depend on the conditions under which the reactor is operating. The excess formaldehyde in the form of an aqueous solution is extracted from the foot of the column and is recovered. The column distillate (i.e., the aqueous mixture of trioxane and formaldehyde) is extracted with a water-insoluble organic solvent, methylene chloride or benzene are examples of the most used, that dissolves the trioxane, whereby the formaldehyde is left in the remaining aqueous phase. The trioxane has to be recovered from these solutions in organic solvents by distillation and subsequent purification and the aqueous solution has to be treated to recover the formaldehyde.
Owing to the multiple possibilities available for carrying out the diverse steps a large number of patents have been published, each aimed at improving one or another aspect of the process. Thus, there are those that use strong inorganic acids, such as sulphuric acid or phosphoric acid, as catalyst of the trimerization reaction and those which use ion exchange resins in acid form, heteropolyacids, zeolites, montmorillonites and other solid catalysts that have silica as base, to mention only a few.
Whatever the catalyst used, the vapors that originate in the reactor are formed by a mixture of trioxane, formaldehyde and water mainly and small amounts of by-products such as methanol, formic acid, methyl formate, methylal, tetraoxane and dioxymethylene glycol dimethylether. The objective now is to separate the trioxane from the remaining products. In most patents this separation is achieved by means of a first step of fractional distillation that allows the vapors at the head of the column to be enriched in trioxane and a subsequent solvent extraction of the trioxane from said mixture. Other separation processes are based on the introduction of an inert gas into the mixture of vapors that facilitates the separation, azeotropic distillations with solvents or extractive distillations with water or glycols (U.S. Pat. No. 3,281,336) that retain the formaldehyde, or scrubbing of the gases at the exit from the reactor with heavy solvents that dissolve the trioxane (EP 0 680 959). There are further separation processes, such as distillation at two different pressures or simultaneous reaction-extraction or with gases in a supercritical state. This multiplicity of processes shows how hard it is to find an optimum process.
It should be pointed out that in all cases, with the exception of the extractive distillation with water or glycols, the trioxane is extracted from the mixture with water and formaldehyde, obtaining a trioxane solution in a solvent that must subsequently be removed in order to obtain pure trioxane. Only extractive distillation pursues the removal of the formaldehyde from the mixture and the release of the trioxane which separates out easily since it forms an azeotrope with the water that contains a high trioxane proportion. This is recovered by crystallization from said azeotropic mixture. It is a process which dispenses with the need for a solvent, whereby it eliminates the environmental problems that said solvents present. Nevertheless, it has an additional problem, which is that the aqueous formaldehyde solution obtained is highly dilute, whereby the formaldehyde has to be concentrated for later recovery thereof. If the process is carried out with a glycol or higher alcohol such as cyclohexanol, the formal obtained has to undergo a pyrolysis process to recover the formaldehyde. Both one case and the other require supplementary facilities and energy that complicate the process and make it more expensive.